This invention relates generally to interface circuits, and more particularly to circuits used to interface between an output signal produced by one type of integrated circuit technology, such as emitter coupled logic (ECL) technology, and an input signal used by another type of integrated circuit technology, such as complementary metal oxide semiconductor (CMOS) technology.
As is known in the art, it is sometimes desirable to provide an interface circuit adapted to operate with a variety of integrated circuit technologies, each of which has been optimally designed to operate with a different power supply. The integrated circuit technologies typically perform a myriad of analog and digital functions. These integrated circuit technologies may consist of devices and circuit structures bases on bipolar, GaAs, or MOS technologies, to name but a few. Quite often, it is required to design a digital processor based on one of these technologies, yet which may accept inputs from another one of the technologies.
One notable interface problem that is very common in current state-of-the-art digital system design is interfacing between the outputs produced by ECL technology and a digital processor made up of CMOS technology. Traditional ECL integrated circuit designs utilize a negative power supply and generate differential output voltages ranging around several hundred millivolts (i.e. the difference in voltage levels between a logic 0 signal and a logic 1 signal is about 400 millivolts). These differential output voltages have an average voltage level, generally referred to as a common-mode level. The common-mode level for ECL technology can range from a negative potential to a positive potential, and can have a magnitude ranging from several volts to ground. CMOS digital integrated circuit technology, on the other hand, is typically designed to operate with a positive power supply (typically V.sup.+ about 5 volts, V.sup.- about ground potential). The input and output logic voltage levels typically have a nominal level (i.e. common-mode level) of 2.5 volts with one logic state represented by 3.5 volts and the other logic state represented by 1.5 volts.
One technique used to interface between these two types of integrated circuit technologies is to shift the power supply reference for either the ECL circuit or the CMOS circuit so that the total system operates around one power supply (either (+) or (-)). However, this is sometimes not possible in some applications. Further, the technique may not be applicable in interfacing between other types of technology.